The present invention relates to a driving-torque distribution control apparatus of a four-wheel drive vehicle, and in particular relates to the driving-torque distribution control apparatus of the four-wheel drive vehicle which controls a driving torque distributed to a rear wheel of the vehicle.
Conventionally, various technologies of controlling the driving torque distributed to an auxiliary driving wheel (the rear wheel, typically) have been proposed. U.S. Pat. No. 5,752,211, for example, proposes a technology that the fuel economy (gas millage) is improved by decreasing the driving torque distributed to the auxiliary driving wheel during steady traveling of the vehicle. U.S. Pat. No. 5,894,904, for example, proposes another technology that increasing of the traveling resistance of the vehicle is avoided by decreasing the driving torque distributed to the rear wheel according to an increase of a speed difference between left and right rear wheels because of a decrease of a vehicle's turning radius. US Patent Application Publication No. 2013/0035832 A1, for example, proposes further another technology that in a specified rotational-speed range of an engine's output shaft in which some noise is possibly generated at a driving-torque transmission system because of pulsation of the driving torque, the driving torque to be transmitted to the auxiliary driving wheel is set at a torque value capable of decreasing the above-described noise properly.
In the meantime, a gripping limit of a vehicle's tire is generally indicated by a friction circle having the ordinate representing a force acting on the tire in a longitudinal direction and the abscissa representing a force acting on the tire in a lateral direction. In a conventional four-wheel drive vehicle, the driving torque was controlled such that the forces on the friction circle act on the both front-and-left wheels, putting emphasis on improvements of the vehicle's movement performance, traveling performance, and the like. That is, in the conventional four-wheel drive vehicle, the driving torque to be distributed to the rear wheel was controlled so that both tires of the front and rear wheels can be used at the gripping limit, thereby making the maximum longitudinal and lateral forces acting on the tires during the vehicle's traveling. However, the above-described conventional control has a tendency that the traveling resistance of the whole vehicle including the front and rear wheels during the turning does not become the minimum, more specifically, that the total sum of loss energy of tire-loss energy and mechanism-loss energy during the vehicle's turning does not become the minimum. That is, the conventional control is not configured such that the driving torque distributed to the rear wheel is controlled so as to minimize the total sum of the loss energy during the vehicle's turning.